Inclusion Criteria:

• - ≥18 years.

• - Diagnosed with any type of ILD.

• - Fluent in Portuguese.

• - Able to provide informed consent.

Exclusion Criteria:

• - Other respiratory diseases.

• - A history of acute cardiac/respiratory condition in the previous month.

• - Present signs of cognitive impairment.

• - Significant cardiovascular, neurological and/or musculoskeletal disease that may limit their participation.

• - Current neoplasia.

• - Other autoimmune diseases (aside from ID).

Functional status is a fundamental, patient-centred and multidimensional outcome for individuals with interstitial lung disease (ILD). Functional status decline is the most frequent impact reported in this population. It is often associated with an increased dependency on others, loss of productivity and premature death. It is also an important predictor of exacerbations, hospitalisations, readmissions, and mortality. Functional status decline is so overwhelming for individuals and society, that it has been recently recognised as an international top priority for patient-centred research in ILD. Yet, little is known how functional status is impaired and progresses in ILD, limiting the help health professionals are able to provide to optimise care. ILD are a highly disabling group of chronic respiratory diseases characterised by widespread inflammation and scarring (fibrosis) of the lung. In people with ILD, the destruction of the pulmonary capillary bed and vasoconstriction, results in impaired gas exchange and circulatory limitation, which can lead to reduced exercise capacity. Exercise capacity may be further limited due to peripheral muscle dysfunction caused by physical deconditioning and drug-induced myopathy from ILD treatment with corticosteroids. Taken together these factors contribute to the high symptom burden experienced. This population, therefore, presents pulmonary (e.g., dyspnoea, cough) and extra-pulmonary (e.g., decreased exercise tolerance, skeletal muscle dysfunction, fatigue) traits and experience a downward spiral of debilitating symptoms and functional status decline. Functional status is defined as the ability to perform normal daily activities required to meet basic needs, fulfil usual roles, and maintain health and well-being. It includes functional capacity, i.e., an individual's maximum capacity to perform daily life activities in a standardized environment; and functional performance, i.e., activities people actually do during the course of their daily life. Adequate assessment of functional status must, therefore, include measures of functional capacity and performance, to identify the presence or absence of decline and inform on preventive and/or rehabilitative strategies. Studies focussing on the comprehensive assessment of functional status in people with ILD using reliable and validated instruments are, however, lacking. Therefore, the primary aim of SUNSET study is to characterize the functional status and explore the determinants of functional status decline of people with ILD. Secondary aims are to: i) To determine the measurement properties of functional status instruments in people with ILD ii) To identify the impact of ILD and the participants' perspectives on functional status through interviews iii) Explore the progression of functional status progression in people with ILD and iv) Develop a multidimensional index, incorporating functional status parameters, to predict mortality in people with ILD. According to the COnsensus-based Standards for the selection of health Measurement INstruments (COSMIN) guideline, a minimum sample size of 100 will be required for task 1. For task 2, number of participants needed for interviews will be determined by data saturation. For task 3, a sample size estimation was performed in GPower to detect a small effect size (eta^2=0.02), with 80% power, 5% significance level and moderate correlation among repeated measures (r=0.5) in a repeated measures analysis of variance. The minimum sample size estimation was 81 participants Considering a dropout rate of 50%, we will aim to recruit 122 participants. This study is a multicentric observational longitudinal research project comprising four tasks. It involves collaboration between institutions, namely the University of Aveiro, along with three healthcare facilities: Centro Hospitalar de Vila Nova de Gaia/Espinho (CHVNG/E), Centro Hospitalar do Baixo Vouga (CHBV), and Centro Hospitalar de Entre o Douro e Vouga (CHEDV). Task 1 aims to validate the measures of functional status. For this, two assessment moments will be performed. The first assessment will be conducted at baseline and will be in line with the routine appointment. The second assessment will be conducted 7 days after baseline. Task 2 will use the information collected in Task 1 and conduct semi-structured individual interviews to characterize the functional status from the perspective of people with ILD. Task 3 will then conduct an observational longitudinal study using the most valid and reliable measures of functional status identified in task 1 and mortality rates will be explored during task 3. For this, patients will be contacted to participate in four timepoints of assessment (at 6, 12, 18, and 24 months) which will be in line with the routine appointment. Table 1 details the measurements performed in each timepoint. If an outcome measure is found to not be valid and reliable during task 1, that outcome measure will not be collected in task 3 nor replaced by another outcome measure. In Task 4, the data collected in Task 3 will be used to develop a multidimensional index incorporating functional status parameters to predict mortality in people with ILD. The study assessments will take approximately 90 minutes and will take place at Centro Hospitalar de Vila Nova de Gaia/Espinho (CHVNG/E), Centro Hospitalar do Baixo Vouga (CHBV), and Centro Hospitalar de Entre o Douro e Vouga (CHEDV). Patients with ILD will be recruited via the pulmonology services at hospitals. Pulmonologists will explain the study briefly and inform the researchers about interested participants. The researchers will provide further information about the study, clarify any doubts and collect the informed consent. Participants will be eligible if they are: i) ≥18 years, ii) diagnosed with any type of ILD, iii) fluent in Portuguese and iv) able to provide informed consent. Participants will be excluded if they have: i) other respiratory diseases ii) a history of acute cardiac/respiratory condition in the previous month; iii) present signs of cognitive impairment; iv) significant cardiovascular, neurological and/or musculoskeletal disease that may limit their participation; v) current neoplasia, vi) other autoimmune diseases (aside from ILD). Once task 1 is finished, included participants will be contacted again to verify if they still match the inclusion/exclusion criteria and are still willing to participate in task 3. Baseline data will be collected: Sociodemographic, clinical characteristics (i.e., smoking habits, vital signs and symptoms), anthropometric (i.e., height and weight to compute body mass index) and general clinical data (i.e., medication, oxygen therapy, non-invasive ventilation, acute exacerbations, hospitalizations and number of hospital admissions in the last month and year, length of stay), as well as prior and follow-up lung function and arterial blood gas will be collected from clinical records for patients' characterization. Mortality and rehospitalizations will be explored during the study period. Peripheral muscle strength, functional status, daily physical activity, self-reported symptoms, functional status, impact of the disease and health-related quality of life. Qualitative data from interviews. Data analysis will be conducted using the Statistical Package for the Social Sciences (SPSS) software, incorporating both descriptive and inferential statistics. To examine changes in outcome measures, data from baseline and subsequent assessments will be compared. Additionally, the interviews will be transcribed and analysed using thematic analysis.